1. Field Of The Invention
This invention relates to circuits for controlling programmable configuration circuitry and, more particularly, to circuitry for protecting a pair of UPROM memory cells against reprogramming once one of the cells has been programmed.
2. History Of The Prior Art
A UPROM based configuration circuit which utilizes a pair of UPROM devices is used in many digital circuits to provide a voltage of a predetermined value which may be used in an associated circuit as a logical one or a zero value. A UPROM device is essentially an electrically programmable read only memory device (EPROM device) with some means of protecting against its condition being changed once it has been placed in a particular condition. Typically the memory device is a floating gate field effect transistor capable of assuming a programmed condition or an erased condition. In the programmed condition, no current flows between the source and drain terminals of the transistor; while in the erased condition, the transistor conducts between these terminals. This type of UPROM circuit is utilized because it allows a programmable voltage value to be provided continuously at its output without drawing a significant amount of current. Consequently, the arrangement allows a given voltage to be maintained without a great deal of power consumption.
Such a circuit includes a pair of UPROM transistors with cross-coupled biasing means and circuitry for placing one or the other of the two UPROM transistors in the programmed state. If a first one of the UPROM transistors is programmed, then one voltage is provided constantly at the output. If the other one of the UPROM transistors is programmed, then another voltage is provided constantly at the output. These voltage values are, as pointed out above, used as logical one and zero values. Typically, such a circuit is programmed to a first output value at the factory and sold to be used for one purpose, or programmed to a second output value and sold to be used for another purpose.
It is possible that both such transistors of the UPROM circuit may be programmed (and consequently non-conducting) rather than just one transistor. For example, a test program may cause advertent programming of the UPROM transistors if the program is written incorrectly. In such a case, the output voltage provided by the UPROM circuit is indeterminate and depends to a great extent upon the characteristics of the individual biasing devices used in the circuit. Since in the present arrangement the voltage value established is the value which is used to configure the UPROM circuit to produce one of two output values which is to be used to establish a level for a logic value used in a digital circuit, a circuit which provides an indeterminate value can cause significant difficulties.
In prior art arrangements utilizing such UPROM devices to provide a selectable output voltage, it has been typical to provide an independent circuit which protects against the unprogrammed one of the two UPROM devices being programmed once the other UPROM device has been placed in the programmed condition. Typically, this independent circuit is a circuit similar to the circuitry which is itself programmed to produce the desired output. Such a circuit uses a number of components and, therefore, adds some substantial expense to the overall circuit. In prior art arrangements, however, the protection circuitry has typically been used to guard against the programming of a number of such UPROM cells; and, consequently, its expense has been relatively small when distributed over the cost of all of the circuits. In the desired use, however, only a single UPROM circuit is to be protected so the relative expense is much greater. Consequently, the use of the prior art protection circuit is less desirable.
Moreover, the prior art protection circuit only comes into operation once the UPROM circuit has been programmed. A time period exists after programming the first UPROM during which no protection exists, and it is possible to program the other UPROM during that period. Such programming would leave the UPROM circuit in the undesirable condition in which an indeterminate output voltage is produced.
For these reasons, it is desirable to provide improved protection against programming of the UPROM memory transistors once the appropriate programmed state for the UPROM circuit has been selected.